The present invention relates to apparatus generating a magnetic field for an installation for the acceleration of electrically charged particles, the particle track of which contains curved and straight sections, comprising windings which generate a magnetic field and of which at least one supplemental winding is provided for focusing the particles on the particle track. Such apparatus are known, for instance, from the publication "Nuclear Instruments and Methods", vol. 203, 1982, pages 1 to 5.
With known smaller electron accelerators of circular shape, which are also called "microtrons", particle energies to approximately 100 MeV can be achieved with normal conducting magnetic field generating windings. These installations can also be realized, in particular, as so-called "racetrack microtrons". The particle tracks of this type of accelerator comprise two semicircles each with a corresponding 180.degree. deflection magnet and two straight track sections (see "Nuclear Instruments and Methods", vol. 177, 1980, pages 411 to 416, or vol. 204, 1982, pages 1 to 20).
If the desired final energy of the electron is to be increased from about 100 MeV to substantially higher values of, for instance, 700 MeV, an increase of the magnetic field is available while the dimensions of the particle track remain unchanged. Such an increase can be achieved particularly with superconducting magnets. If, however, low-energy electrons are injected with a very weak magnetic field into a microtron which, in addition, comprises superconducting magnet windings, a number of possible field error sources must be noted in order to keep the electron losses low during the acceleration phase, since at the start of this phase, the field level for electrons injected at low energies of, for instance, 100 keV, is only about 2.2 mT with a radius of curvature of the accelerator of, for instance, 0.5 m. With such low magnet field strengths or also with high field change rates, the danger then exists, however, that the field error limit may be exceeded because of field distorting error sources. In order to be able to guide an electron beam by weak focusing, a field accuracy .DELTA.B/B.sub.0 of about 10.sup.-3 would be required in the above-mentioned case; this means that the field would have to be adjustable with an accuracy of about 0.002 mT at the start of the acceleration phase. Then, however, external fields such as the Earth's field with 0.06 mT or fields of magnetizable, i.e., para-, ferri- or ferromagnetic parts of the magnetic apparatus itself can be the cause of undesirable field distortions. Also, eddy currents in metallic parts of the magnetic apparatus or in its conductors can lead to such disturbances. In addition, shielding currents in the conductors of the superconducting winding or so-called frozen magnetic fluxes in these conductors can represent such disturbance sources.
It has been attempted to eliminate difficulties due to such interference field sources, for instance, by shielding or compensating the interference fields. Thus, a shielding effect by means of a flux return of iron has been attempted in known electron accelerators with normal-conducting copper coils. In additon, also laminating of the iron yokes of the field-generating magnets for suppressing the development of eddy currents is known. Optionally, a field reversal can also be carried out in order to traverse the hysteresis curve of the iron of the magnetic apparatus in a reproducible manner.
If the particles are to be injected into the particle accelerator track with a relatively low energy, a further difficulty results if relatively large particle currents are to be generated, since then the repulsion forces acting between the individual particles, i.e. the space charge forces are relatively dominant; i.e., the particle current attempts to diverge accordingly. One is therefore forced to provide additional measures for focusing the particle beam. In the electron accelerator found in the literature reference "Nuclear Instruments and Methods" mentioned above, the 180.degree. deflection magnets comprise, with a main winding generating a dipole field, also a supplemental winding focusing the particles onto the particle track. In addition, a focusing solenoid system is provided in the region of the straight track sections. However, in the known magnetic apparatus, the normal conducting deflection magnets surround with their iron yokes the respective curved section of the particle track for reasons of the desired field accuracy, so that the synchrotron radiation occurring there cannot be utilized.
Due to the disturbing effects on low energy particle beams resulting if superconducting deflection magnets are used, the particles, in known accelerators, are generally injected only at a higher field level, i.e., with higher energy, since then, the mentioned disturbing effects are only of smaller or secondary significance. Such a mode of operation of the accelerators, however, requires appropriate preaccelerators and is therefore accordingly costly.